PROJECT SUMMARY In the United States and throughout the world, alcohol use is a near universal societal and cultural norm. Fortunately, the majority of people who consume alcohol do not develop an alcohol use disorder. This suggests the existence of a homeostatic pathway(s) that protects individuals from developing and suffering from alcohol addiction. Data from my laboratory group and others suggest that brain-derived neurotrophic factor (BDNF), a growth factor involved in structural and functional neuronal plasticity, gates the transition from moderate alcohol drinking to excessive uncontrolled consumption. The goal of this proposal is to identify the cell type and circuit that BDNF signals through to gate excessive drinking. Moderate consumption of alcohol increases BDNF mRNA levels in the dorsolateral striatum (DLS) of mice, which `brakes' further alcohol intake, whereas excessive and repeated alcohol drinking disrupts the homeostatic increase of BDNF in the DLS, transitioning the animal towards excessive drinking. BDNF in the DLS acts to suppress alcohol intake by activating the effector genes, preprodynorphin and dopamine D3 receptor. The majority of DLS neurons are medium spiny neurons (MSNs) which express either dopamine D1 receptor (D1R) or dopamine D2 receptor (D2R). The neuronal subtype(s) that BDNF signals through to prevent excessive alcohol intake is unknown. Aim 1 will determine if BDNF is acting upon D1R and/or D2R MSNs in the DLS to keep alcohol intake in moderation. Because dynorphin and D3 are constrained to D1R MSNs, I hypothesize that BDNF is acting via the D1R pathway to gate excessive alcohol consumption. To test this hypothesis I will use transgenic mice strains that express Cre-recombinase in specific MSN subtypes and a viral vector strategy to delete the BDNF receptor, TrkB, in DLS D1R or D2R MSNs, in combination with voluntary drinking paradigms. The major source of BDNF in the striatum is the cortex. I recently found that excessive alcohol drinking reduces BDNF expression in the orbitofrontal cortex (OFC), an area of the brain implicated in associative learning and regulating reward value. I further found that overexpression of BDNF in the OFC prevents the development of excessive alcohol consumption, and that BDNF released from the OFC activates the BDNF/TrkB pathway in the DLS. Therefore, I hypothesize that BDNF in the OFC ? DLS circuit is preventing excessive alcohol drinking, and that breakdown of BDNF in this pathway drives excessive alcohol consumption. Aim 2 will test the role of BDNF specifically in the OFC ? DLS pathway in regulating alcohol drinking behaviors. To test this hypothesis I will use a combination of viral vectors (Cre-DIO expression system) to knockdown or overexpress BDNF in the OFC ? DLS circuit and assess alcohol drinking behaviors. Collectively, this research will shed light on BDNF signaling and alcohol's actions in the CNS, and innovate on current research by investigating the actions of growth factors in specific neuronal circuits that gate drinking.